Obtaining exact time information of hits in silicon strip sensors read out by the APV25 front-end chip

Silicon strip detectors feature good spatial resolution thanks to the high granularity of the strips. However, they do not provide useful time information due to the long shaping time of the subsequent amplifier. In a system with many off-time background hits and high occupancy, such as the innermost layer of the BELLE Silicon Vertex Detector (SVD), this leads to many measured hit points which do not belong to the triggered event. The APV25 front-end chip was developed for the CMS Silicon Tracker and operates at 40 MHz with a shaping time of 50 ns . In CMS, a “deconvolution” is performed on the chip which narrows the sampled waveform down to one clock cycle, but requires synchronicity between beam and clock. Hence, it cannot be applied in BELLE because of its quasi-continuous beam. Nevertheless, we have developed a method to measure the peak time of the shaping curve with a precision of a few nanoseconds using the multi-peak mode of the APV25, where three (or 6,9,12,…) consecutive samples along the shaping curve are recorded with each trigger. We present details and variations of the method, results obtained in a beam test and the intended implementation of the time finding based on FPGAs.